This invention relates to a method of operation of a nuclear reactor which is provided with nuclear fuel elements inserted in its core, control rods and a mechanism for controlling the power of the reactor by neutron moderator, neutron absorber, neutron reflector etc. arranged in the core or its surrounding, particularly to a method of operating the nuclear reactor for raising load following performance of the nuclear reactor without failure or damage risk of the nuclear fuel elements.
Nuclear reactors which can follow a load increasing or decreasing in accordance to electric power demand changing between working days and holidays, days and nights are in great need. Conditions under which the fuel elements are used during the load following operation are very severe, compared with ones when operated under a rated power for a long time. Therefore, at the present day the reactors are under the severe operating conditions. One of the nuclear reactors, a boiling water reactor (referred to BWR hereinafter) which is now in practice is not exceptional.
Fuel elements for the BWR each usually comprise pellets which each are about 10 mm in its height and stacked in a column and a cladding of about 4 m in its length and about 11 mm in its inner diameter which contains therein the stacked pellets, and is sealed at both ends. The diameter of the pellets and the inner diameter of the cladding are controlled precisely so that a constant diameteral gap is made between the pellets and and cladding. Usually, the gap is determined to be such a size (several 10 .mu.m to several 100 .mu.m) that thermal expansion of the pellets can be absorbed well enough. Therefore, if the pellets are subjected only to the thermal expansion, pellet-cladding interaction (hereinafter referred to PCI) does not occur. Much experimental data, however, show that PCI occurs from a power much lower than a rated power even if the fuel element is designed and manufactured thus.
PCI occurs as follows. The fuel element as inserted in the core has a constant diametral gap between the cladding and the pellets. When the power of the fuel element is increased, a great temperature gradient (2000.degree.-3000.degree. C./cm) is produced in the pellet to induce great thermal stress. The pellet is broken into small pieces by the thermal stress. By thermal elastic energy relieved then, the small pieces of the pellet are moved toward the cladding. This breakage occurs at small linear heat generation rate of 2 to 3 kw/ft. In this stage, the small pieces of the pellet have sufficient freedom so that if their movements are limited by the cladding, they may be restored while being mutually displaced. Therefore, stress which the cladding sustains from the pieces of the pellet is very small.
As the power of the reactor is further increased, the breakage of the pellet progresses further, an amount of thermal expansion of the pellet increases so that the pieces of the pellet are radially pushed away. As a result, the pieces of the pellet are meshed with each other not to be shifted. Thus, the pieces of the pellet and the cladding are brought into very hard contact, and strong stress is applied to each other. It is thought that the PCI thus occurs. The linear heat generation rate at this time is called PCI starting power.
In a range above the PCI starting power, when the power of the reactor is rapidly increased, an amount of movement of the pieces of the pellet to the cladding due to rapid thermal expansion becomes strain of the cladding so that the cladding is forced to deform with a large strain speed and a large strain amount. If the power increase is slowly carried out, however, the pellet creeps by restraint force of the cladding during the slow increase of the power so that the pieces of the pellet as a whole are restored to the central portion. Stress induced in the pellet at this time is small as compared with one induced during the rapid power increase. Thus, it is said that in a linear heat generation rate above the PCI starting power, the larger the power increase speed becomes, generally the higher probability of breakage of the cladding becomes.
If the cladding is broken, fission products with strong radioactivity produced in the pellet are released into coolant. In this case, it may become difficult to continue operation of the reactor.
In order to avoid this risk, a conventional method of operating BWR which is carried out hitherto is described hereinafter. According to the method, until a linear heat generation rate of the fuel element reaches P.sub.1 =8 kw/ft which is described for example in U.S. Pat. No. 4,057,466, power increase of the reactor is effected by control rods at a speed almost unlimited. When the power is raised above P.sub.1, it is done slowly by controlling the quantity of flow of recirculating coolant at a speed of below P.sub.2 =0.06 kw/ft/hr. When power of the reactor reaches a rated power (kw/ft), the rated power is held constant for 12 hours. This operation pattern is called a preconditioning operation. After this preconditioning operation, below the rated power, it is possible to increase or decrease the power somewhat at any speed. Even during the time, above the linear heat generation rate of 8 kw/ft, power increase by withdrawal of control rods is forbidden. Further, it is necessary to effect the preconditioning operation when part of the fuel elements used is exchanged for new one, or a pattern of the control rods is changed.
A control rod drive mechanism of the BWR adopts a notch type drive mechanism which pull out the control rods one by one, and withdraws in an instant by one notch (about 20 cm) every time. When the control rods are fully inserted in the core, the power of the reactor is depressed to a low power. As the control rod is withdrawn by one notch, the power increases by a value corresponding to the amount of the withdrawal of the control rod. At this time, in a low portion of the control rod, the fuel element raises power at a very high rate of 2 to 6 kw/ft/hour.
On the other hand, the power of the reactor is raised by controlling the quantity of flow of the coolant. Namely it is carried out by increasing moderation of neutrons by decreasing an amount of void ratio in the coolant which is increased by controlling the quantity of flow of the coolant. The decrease of the void ratio occurs evenly in the core, so that no partial increase of the linear heat generation rate comes about. The quantity of flow of the coolant can be regulated continuously. Therefore, very moderate power increase can be effected.
In BWR, in order to effect uniform burnup of the fuel element in the core, it is necessary to change an insertion patern of the control rods every fixed period. According to the above-mentioned method, the preconditioning operation must be carried out every the change of the insertion partern. For the preconditioning operation, one to three weeks are needed. Therefore, for this period, the reactor is impossible to effect operation to load following. Namely, for this period, the reactor can not be operated according to the electric power demand.
Accordingly, improvements on the operation method of the reactor is desired.